Conventionally, as a casting process of a non-ferrous metal such as aluminum or an aluminum alloy, a non-ferrous metal manufacturing industry has widely used, for example, a casting process by a so-called gas pressurized hot-top casting mold as disclosed in the Patent Document 1 (JP 54-042847 A) and Patent Document 2 (JP 63-154244 A) below. According to the gas pressurized hot-top casting mold, for example, as illustrated in FIGS. 10 and 11, a molten metal M of aluminum coming out of a hot-top 20 made of a refractory heat-insulating material is directly passed to a passage portion 30 formed in a mold (die) body 10 and at the same time, the molten metal M is forcibly cooled by cooling water W blown out of the mold body 10 to be continuously solidified into a rod-shaped billet B.
As illustrated in FIG. 11, a lubricating oil blow-out hole 40 and a gas passage hole 50 are provided on the upper end of the wall surface of the molten metal passage portion 30 of the mold body 10. When the molten metal M passes through the molten metal passage portion 30, lubricating oil and gases such as inactive gases and air are blown in from the lubricating oil blow-out hole 40 and the gas passage hole 50. This allows the molten metal M to smoothly pass (cast) through the molten metal passage portion 30 with less contact and friction of an inner surface thereof, which can smooth the surface shape of the billet B.
By the way, as illustrated in FIG. 11, the mold for implementing the gas pressurized hot-top continuous casting process includes a refrigerant passage 60 in the mold body 10 so as to forcibly cool the entire mold by the refrigerant (cooling water) W flowing through the refrigerant passage 60. However, according to the conventional mold, the deep annular grooves 70 for supplying a lubricating oil and a gas are annularly formed along the molten metal passage portion 30 of the mold between the refrigerant passage 60 and the lubricating oil blow-out hole 40 and the gas passage hole 50. These grooves 70 act as a heat-insulating layer, thereby preventing the portions of the lubricating oil blow-out hole 40 or the gas passage hole 50 from being cooled sufficiently. Moreover, since the refrigerant passage 60 in the mold body 10 is formed into a rectangular shape in the cross section as illustrated, a part of the refrigerant W flowing through the refrigerant passage 60 is retained at corner portions thereof, thereby preventing effective cooling of the upper portion of the molten metal passage portion 30, which requires heat exchange for solidification.
For this reason, when the temperature of the mold body 10 rises with casting of an alloy with a high molten metal pouring temperature or with a high casting speed, the molten metal cooling capability of the mold reduces, and the surface of the billet B may be in a state of a so-called gas skin. Further, the lubricating effect between the molten metal M and the molten metal passage portion 30 reduces, and the friction between the molten metal passage portion 30 and the molten metal M increases. As a result, the solidified metals and oxides are attached to the surface of the molten metal passage portion 30 and the surface of the billet B tends to be susceptible to a casting defect called shrinking.
Further, since a reduced cooling capability of the mold body 10 reduces the strength of a solidified shell generated from the molten metal M by cooling the mold body 10, as a result, the solidified shell cannot withstand the friction with the molten metal passage portion 30. This causes a problem in that the solidified shell is damaged to be broken out, thereby preventing casting. As illustrated in FIG. 11, after the lubricating oil and the gas supplied from the lubricating oil blow-out hole 40 or the gas passage hole 50 to the molten metal passage portion 30 reach the meniscus portion space S, with the passage of the molten metal M, they advance along the wall surface of the molten metal passage portion 30 and pass downward of the molten metal passage portion 30.
At this time, as the temperature of the mold body 10 rises, a stress from the lubricating oil expansion of the annular lubricating oil supply groove 70 and the thermal expansion of the mold body 10 causes an excess supply of lubricating oil, which is blown out over the molten metal M. Then, the lubricating oil is gasified to cause an excess supply of pressurized gas. The change of the pressurized condition by gas may cause an excessive change of a space (meniscus portion space) S formed between the upper portion of the molten metal passage portion 30, the hot-top 20, and the molten metal meniscus portion m, thereby deteriorating the quality of the billet B.
More specifically, when the gas pressure inside the meniscus portion space S exceeds the molten metal pressure due to the gasification of the lubricating oil, the meniscus portion space S is enlarged and there may occur a phenomenon (bubbling) where a gas and a gasified lubricating oil in the meniscus portion space S escape from the molten metal passage portion 30 to the hot-top 20 side. When such a bubbling occurs, the oxide inclusions or films are generated, which are caught in the surface layer portion of the billet B, thereby causing a surface defect or internal defect of the billet.
If such a defect remains in the final product, the mechanical characteristics of the product are reduced, a forging crack defect at forging occurs, or a visual defect in alumite occurs. Further, if such a bubbling occurs, the meniscus portion space S vanishes momentarily, and the molten metal M may be stuck in the lubricating oil blow-out hole 40 and the gas passage hole 50, where the molten metal M may be solidified or fixed so as to block the holes. As a result, since the meniscus portion space S is not formed later, a big cast skin defect may occur, thereby causing a billet defect.